Vented, gas-fired appliances relying on a natural draft for the removal of products of combustion are equipped with a draft hood which isolates the combustion chamber from excessive updraft or backdraft in the vent. The conduit between the draft hood relief opening and the outside is permanently open and therefore causes heat loss, especially during cold and windy weather.
Automatic vent dampers have been designed to reduce this loss. Examples are the thermally controlled vent dampers described in U.S. Pat. Nos. 3,228,605 and 3,510,059. These automatic, thermally controlled vent dampers are installed between the draft hood and the vent connector. They are open during the operating phase of the appliance and are closed during standby and shutdown periods.